1. Field of the Invention
This invention relates to a wavefront aberration compensation element, an optical pickup, and an optical disk apparatus for accommodating light sources of various wavelengths.
2. Description of the Related Art
An optical disk apparatus records information on a recording surface of an optical disk with spiral or concentric tracks formed thereto, and performs operations of recording/reproduction/erasing information based on the light directed to and reflected from the recording surface of the optical disk.
Typically, an optical system of an optical pickup includes a light source 101 and an objective lens 102 as shown in a schematic diagram of FIG. 9. In order to control the fine spot on an information recording medium 103, the optical system of the optical pickup is provided with a lens actuator 104 for executing focus and track control of the objective lens 102. Numeral 105 is a detection/separation part (beam splitter) for separating reflected light of the information recording medium 103 from incident light, and guiding the reflected light to a light receiving element 106.
As for the kinds of optical disks provided in recent years, there are, for example, a DVD (Digital Versatile Disc) having a considerably larger memory capacity than a CD (Compact Disc), and a recording/reproduction disk using blue laser. Two standards of the recording/reproduction disks using blue laser are proposed, one having a substrate thickness of 0.1 mm (Blu-ray) and the other having a substrate thickness of 0.6 mm (HD-DVD). In recording/reproducing information, a light source with a wavelength of 785 nm is used for the CD, a light source with a wavelength of 660 nm is used for the DVD, and a light source with a wavelength of 405 nm is used for the recording/reproduction disk using blue laser. Relationships between the thickness of a disk substrate, the wavelength of a light source, and the NA (Numerical Aperture) of an objective lens are shown in Table 1.
TABLE 1DVDCDHD-DVDdisk 0.6 mm 0.6 mm 1.2 mmthicknesslight source 405 nm 660 nm 785 nmwavelengthobjective0.6-0.70.6-0.650.3-0.55lens NABlu-raydisk 0.1 mm 0.6 mm 1.2 mmthicknesslight source 405 nm 660 nm 785 nmwavelengthobjective0.850.6-0.650.3-0.55lens NA
Meanwhile, a typical objective lens is designed to eliminate wavefront aberration created by the thickness of the disk substrate. Accordingly, wavefront aberration is caused by disk substrates having different thicknesses and/or optical pickups for different wavelengths. Therefore, recording/reproduction/erasing cannot be suitably performed.
For example, as shown in FIG. 10, a wavefront aberration is created in a case where a parallel light of a DVD is incident on an objective lens corresponding to the 0.1 mm disk thickness of the Blu-ray standard. Typically, in order to reduce the spot diameter of a light condensed with an objective lens, it is preferred to restrict the wavefront aberration to an rms of 0.07 λ or less (Marechal Criterion). However, the wavefront aberration shown in FIG. 10 is a large wavefront aberration of 0.7 λ. With such an optical pickup, the spot diameter cannot be reduced, and recording/reproduction/erasing cannot be suitably performed.
In order to solve this problem, the following conventional technologies, enabling recording/reproduction/erasing by selectively compensating for wavefronts with respect to disks having different thicknesses, are proposed.
As shown in Japanese Laid-Open Patent Application No. 10-334504, the first conventional example uses a wavelength selection phase plate. This example has one optical system for emitting and receiving a laser beam with a prescribed wavelength and another optical system for emitting and receiving a laser beam with another prescribed wavelength. Furthermore, this example is provided with a light combining/separating part (interference filter) for combining the laser beams emitted from the semiconductor lasers of each of the optical systems and thus separating the light reflected from an optical recording medium, to thereby allow light to be guided to either one of the optical detectors in the optical systems. Furthermore, this example is provided with a wavelength selection phase plate disposed between the interference filter and an objective lens for changing the phase distribution of the transmission wavefront with respect to either one of first and second semiconductor lasers. Accordingly, this example enables reproduction of disks having different thicknesses (substrate thicknesses). Furthermore, this example enables a satisfactory S/N and jitter result to be obtained during reproduction. Furthermore, this example enables an optical head apparatus to obtain a sufficient output and a peak strength during recording.
As shown in Japanese Registered Patent No. 02895150, the second example uses a liquid crystal element for compensating for wavefront aberration. This example is an image formation optical system having an aberration compensation mechanism which obtains compensation signals by detecting the amount of aberration, and controls the aberration compensation mechanism based on the detected amount of aberration. In this example, a liquid crystal element, being divided into panels, is used as the aberration compensation mechanism, in which liquid crystal panels are driven in accordance with detected aberration patterns. This allows wavefront aberration of the image formation optical system to be compensated for.
As shown in Japanese Registered Patent No. 03236203, the third example performs spherical aberration compensation by using divergent light. In this example employing an optical head with light sources of different wavelengths, the distance between a light source of a long wavelength and an objective lens is shortened, so that light is incident on an objective lens in a diverging manner, to thereby create a negative spherical aberration. Since the objective lens is designed to reduce the spherical aberration with respect to a light source of a short wavelength, a positive spherical aberration is created when a short wavelength light is incident to the objective lens. Accordingly, the spherical aberration of the optical head can be reduced by the positive spherical aberration and the negative spherical aberration offsetting each other, thereby obtaining an optical head having little spherical aberration with respect to both light sources of long and short wavelengths.
Next, problems of the above-described conventional examples are described in a case of compensating for wavefront aberration for a DVD by using an objective lens with a substrate thickness of 0.1 mm according to the Blu-ray standard.
First, the first example using the wavelength selection phase plate is described. FIG. 11 is a diagram showing four levels of spherical aberrations before and after compensation. As shown with the line indicating “before compensation”, the levels of the phase distribution compensation amount of the transmission wavefront is to be minutely defined in order to compensate for large wavefront aberrations. However, since the pupil diameter is constant, minutely defining the levels narrows the width of each level and causes difficulty in manufacturing (processing). Furthermore, in the wavefront aberration after compensation, a serrated wavefront aberration remains and an rms value of 0.07 λ or less cannot be obtained.
In addition, with such an optical element being manufactured mainly with glass or molded plastic, the refractive index is drastically changed by wavelength changes due to fluctuations in wavelengths and changes in atmospheric temperature, and changes in wavefront aberrations (chromatic aberrations) are caused by phase difference. Furthermore, with such a phase conversion element, newly created wavefront aberrations due to axial deviation between the phase conversion element and the optical system cannot be compensated for.
Next, the second example using the phase conversion liquid crystal element is described. For example, in a case where the double refraction of a liquid crystal element is 0.2, the thickness of the liquid crystal layer is 5 μm, and the applied voltage is 4V, the obtained phase difference is 0.7 μm (=1 λ at 660 nm) (applied to a liquid crystal of an optical disk head in FIG. 12). Considering that the refractive index is 0.2 to 0.25, and that the power source of the optical pickup is mainly 5V, the wavefront aberration compensation of the liquid crystal element has a limit of 1 λ. That is, the compensation for the wavefront aberration using the liquid crystal element is unsuitable for large aberrations.
Next, the third example of compensating for spherical aberration using divergent rays is described. By setting the divergent point of the DVD for creating divergent rays with respect to the objective lens, the spherical aberration of the DVD can be reduced as shown in FIG. 13. However, since the divergent point of the DVD is fixed, the layout of the optical system is restricted. Furthermore, a residual aberration is caused from differences in magnification of the optical system, and the rms value of 0.07 λ or less cannot be obtained.